Automatic interface for gas chromatograph-mass spectrometer system

ABSTRACT

AN ANALYTICAL SYSTEM IS DESCRIBED INCLUDING A COMBINATION OF A GAS CHROMATOGRAPH AND A MASS SPECTROMETER ALONG WITH AN AUTOMATIC INTERFACE APPARATUS FOR RENDERING THE TWO DISSIMILAR COMPONENTS HIGHLY COMPATIBLE IN A CONTINUOUS FLOW QUANTITATIVE-QUALITATIVE ANALYZING SYSTEM. THE INTERFACE INCLUDES AN AUTOMATIC CONTROL CIRCUIT FOR DETECTING THE OUTPUT OF THE GAS CHROMATOGRAPH AND SELECTING FROM EACH GC PEAK A PREDETERMINED QUANTITY OF SAMPLE FOR PRESENTATION TO THE ION SOURCE OF THE MASS SPECTOMETER.

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United States Patent O1 3,563,083 Patented Feb. 16, 1971 hcc 3,563,083AUTOMATIC INTERFACE FOR GAS CHROMATO- GRAIH-MASS SPECTROMETER SYSTEMHanspeter Benz, Palo Alto, Calif., assignor to Varian Associates, PaloAlto, Calif., a corporation of California Filed Jan. 15, 1968, Ser. No.698,019 Int. Cl. G01n 31 08 U.S. Cl. 7323.1 6 Claims ABSTRACT OF THEDISCLOSURE This invention relates in general to analytical instrumentsfor deriving qualitative as well as quantitative information from agaseous material and more speciiically to an apparatus for enabling themarriage of two different types of investigative instruments into aprecision analytical system so as to facilitate the highly sensitiveinvestigation of certain materials.

PRIOR ART Among the most useful and sensitive instruments available tothe chemist are the lgas chromatograph (GC) and the mass spectrograph.The basic characteristic of the gas chromatograph is its ability toseparate and make quantitative measurements. Its qualitative measurementcapability however, is limited and is dependent on calibration andfurther experimentation. The principal advantage of the massspectrometer resides in its great sensitivity and the amount ofqualitative information which it is capable of providing.

These differing capabilities of the respective devices are the factorswhich lead to the desirability of a combination of the methods of gaschromatography and mass spectrometry in a single analytical system.Fortunately, the two devices have an important facet in common; theamount of sample material which each can handle is in roughly the samerange, i.e., from nanograms to fractional milligrams.

However, the two devices diifer in other important aspects. The gaschromatograph for example, operates at near atmospheric pressure whilethe mass spectrometer operates at a high vacuum. Furthermore, thecarrier gas and sample mixture in the chromatograph ows at a rate far inexcess of the permissible ilow of material in the ion source of the massspectrometer. These dissimilarities would tend to make the two devicesincompatible were it not for the use of a suitable interface apparatusallowing the automatic selection of only a portion of the output of thechromatograph to be supplied to the ion source of the mass spectrometer.Various interface devices have been developed which allow a portion ofthe ecluent of a gas chromatograph to be directly introduced into a 'gasanalyzer. These devices are disclosed in the articles: Quantitative andQualitative ionization Detector for Gas Chromatography, pp, 195-205,Proc. Instr. Soc. Am. 1961; Use of a Mass Spectrometer as a Detector andAnalyzer for Eluents Emerging from High Temperature Gas LiquidChromatograph Columns, pp. 759- 764, Anal. Chem., 36(4) April 11964; andHigh-Resolution Mass Spectra of Compounds Emerging from a GasChromatograph, pp. 1135-7, Anal. Chem., 36(6) May 1964.

Another apparatus having substantial advantages over the above devicesis disclosed in the copending Llewellyn application Ser. No. 511,756 nowPat. No. 3,455,092 'filed Dec. `6, 1965 now Patent 3,455,092 issued July15, 1969 and assigned to the assignee of the present invention. Thisapparatus includes a permeable membrane type separator which serves thedual function of separating the material to be analyzed from the carriergas (thus reducing the ilow rate) while at the same time enabling theintroduction of the sample material into the mass spectrometer at agreatly reduced pressure.

In subsequently iled applications Ser. Nos. 626,193, and now abandoned,626,194 now Pat. No. 3,398,505 and 626,196 now Pat. lNo. 3,471,692issued Oct. 7, 1969 (all iiled on Mar. 27, 1967 and assigned to theassignee of the present invention) various improvements to the abovementioned Llewelyn invention are disclosed. This invention relates tostill a further improvement to the previously disclosed gas analysissystem, however, the present invention is more particularly directedtoward an automatic control system for enabling the extraction of apredetermined quantity of sample material from each sample peak in theeiliuent of a chromatographic column or any other suitable source ofgaseous material.

OBJECTS The principal object of this invention is the provision of anautomatic interface for increasing the utility and compatibility of agas chromatograph-mass spectrometer system.

Another object of the invention is the provision of a means forautomatically selecting from the output of a gas chromatographic columnpredetermined quantities of sample material for presentation to theinput of a mass spectrometer.

A further object of the invention is to provide a sensitive detectionand control circuit for operating a valve means which diverts from theoutput iiow path of a gaseous material separating apparatus :into theinput flow path of a gas analyzer means or fraction collecting means apredetermined quantity of sample material.

A still further object of the invention is to provide a sample selectioncontrol circuit for automatically opening a valve means in the fllowpath between a gas chromatograph and a mass spectrometer upon sensing aquantitative peak in the output of the gas separating means, integratingthe output until a predetermined quantity of gas has passed through saidvalve means and then automatically closing said valve means.

Other objects and advantages of the present invention will becomeapparent after having read the following detailed disclosure of apreferred embodiment shown in the drawings.

DRAWING FIG. 1 is a schematic diagram of a gas analysis systemincorporating the present invention,

FIG. 2 is a chromatograph exemplary of the output of the gaschromatography of FIG. l.

FIG. 3 is a sequencing chart indicating the operational sequence of thecontrol system components with relation to the exemplary chromatogram ofFIG. 2.

DESCRIPTION In FIG. 1 there is shown a gas analysis system including asample selection and control circuit in accordance with the presentinvention. The system includes a source which supplies a continuous owof a suitable carrier gas to the gas chromatograph 12. Typical carriergases include the permanent gases such as He, H2, N2, Ar. At the input11 to the column of the gas chromatograph the sample gas is mixed withthe carrier gas. As the mixture passes through the column the variousgaseous constituents are time separated within the ow stream and may bedetected at the column output as a series of peaked curves (GC peaks) ofvarying amplitudes and durations such as is depicted in FIG. 2. Therespective areas under these GC peaks are representative of the relativeproportions of the various constituent gases in the sample. Depending onthe gaseous materials in the sample and the type of column packing,etc., the peaks may be separated by as little as a few seconds or by asmuch as several minutes. In some instances where the separation of twoor more constituents is less distinct, an unresolved, plural maxima peak`will be detected.

The output of the gas chromatograph 12 is connected through a splitter14 to a valve means 16 including a spool 17 which selectively directsthe flow stream through either a vent port 18 or through a conduit 20which communicates with the input of a fluid separating means 22. Asuitable structure for the valve means 16 is disclosed in theaforementioned applications Ser. Nos. 626,- 193 and 626,196.

In the normal operating position (shown in dashed lines in the drawing)spool 17 of the valve means 16 directs the flow stream fromchromatograph 12 out through vent 18 where it may be collected,exhausted or introduced into other analytical equipment. When the valveis actuated into its other position, las shown in solid lines in thedrawing, the iiow stream is directed through a conduit 20 to a separator22, which may be of the 2- stage permeable membrane type `disclosed inthe aforementioned application Ser. No. 511,756. As the gaseous mixturepasses through the separator 22 the excess material is vented while aportion of the sample material is separated out to be introduced intothe ion source of the mass analyzer 24. A suitable vacuum pump 26 isprovided for evacuating the analyzing chamber of the analyzer 24 down toa pressure of approximately 10-6 torr.

In order that a continuous low of gas be provided to the input of theseparator 22 a bypass conduit 13 is connected from the gas source 10 tothe valve means 16.

Referring again to FIG. 2 it will be noted that the form of each sampleGC peak differs in some respect from the other GC peaks. Since thecalibration of the output of the mass spectrometer 24 depends on thequantity of sample input thereto it is desirable that the quantity ofsample material selected from each peak for introduction into the massanalyzer be the same. This necessi tates opening the valve 16 for adifferent time At for each GC peak due to the disimiliarity in thecharacteristics of the respective peaks.

One method of selecting a sample is to manually control the valve 16 inaccordance with the projected or observed output from the column.Another method is to provide a timing means which causes the spool 17 ofvalve 16 to be placed in its sample selecting position for apredetermined period of time. By referring to the exemplary curves shownin FIG. 2 it will be apparent however, that such methods cannotreasonably be expected to select, with any degree of accuracy,substantially equal quantities of sample material from the non-similarpeaks.

Returning now to FIG. 1 there is shown a valve control system, inaccordance with the present invention, which provides a means forautomatically selecting from each GC peak an equal quantity of material.The Splitter 14, which may be of the type disclosed in copendingapplication Ser. No. 666,618 filed Sept. 1l, 1967 now Pat. 3,498,027issued Mar. 3, 1970 and assigned to the assignee of the presentinvention, diverts a small percentage (approximately 1%) of the owstream to a suitable detector apparatus 27 which may, for example, ,be aflame detector 27. The flame detector 27 produces an electrical outputsignal which varies in accordance with the quantitative distribution ofthe sample material in the efliuent iiow stream of the chromatograph 12(see FIG. 2).

By means of a lead 28 this electrical signal is supplied to a valveactuating and control circuit in accordance with a preferred embodimentof the invention which is comprised of a minimum detector 31), a maximumdetector 36, a valve actuator 42 and switching means S4, an integrator48, a signal comparator 50 and a control amplier 52. The signal fromdetector 27 is fed directly to the input of the minimum detector 30, themaximum detector 36 and through the switch means S1 to the integrator48.

For purposes of illustration the minimum detector 30 is shown in itsmost basic schematic form including an operational amplier 31, a diode32, a resistor 33 and a capacitor 34. The operational amplifier 31 is ofthe type that produces an output signal only when the voltage on itsinput leads are not substantially equal. When a signal appears on lead28 and the switch S1 is closed, as shown, connecting the capacitorthereto through resistor 33, the voltage on the capacitor 34 will followthe signal voltage so that the potential on both of the input terminalsof amplifier 31 are approximately equal and no output signal isgenerated thereby. When the switch S1 is opened the capacitor willfollow only excursions of signal voltage in the negative direction dueto the polarity of the diode 32 and upon the first excursion in thepositive direction, as at points c, f, etc. in FIG. 2, the voltagesapplied to the input of the amplifier 31 will differ and an output willbe generated thereby. This output is connected through a lead 35 so asto open switch S2 and close switch S3 as illustrated in the drawing.

The maximum detector 36 is schematically illustrated in the manner ofminimum detector 30 previously described except that the polarity of thediode 37 is reversed so that the operational amplifier is sensitive tosignal excursions in the negative direction when the switch S2 is in itsopen position as shown. The output of amplifier 38 is connected througha lead 39 to switch S1 where it acts to close switch S1 when the signalin line 28 begins to decrease in magnitude, as at points a, d, g, etc.in FIG. 2 of the drawing.

The output of amplifier 38 is also connected to a triggerable powersupply means 40 of valve actuator means 42 which is turned ON by asignal generated by amplifier 38. The output of power supply 40 isconnected through switch means S3 to a solenoid 44 which, whenenergized, causes the spool 17 of valve 16 to be displaced into itsupper position allowing sample gas to ow into separator 22. A springmeans 46 is provided for returning the spool 17 to its lower positionwhen switch S3 is opened.

As the spool 17 is displaced upwardly in response to the detection ofpoints a, d, g, etc. in FIG. 2, switch means S4 is closed connecting theinput of integrator 48 to the output of llame detector 27. During thetime that switch S1 is closed integrator `48 effectively provides asummation of the area under each peak between the points a-b, etc.,which corresponds to the shaded areas shown in FIG. 2. The output ofintegrator 48 is connected to a comparator 50 which is presettable so asto generate an output signal when the integration has reached apredetermined value corresponding to the amount of sample which is to besupplied from each GC peak to separator 22 for transmission to massspectrometer 24.

The output signal from comparator 50 causes control amplifier 52 togenerate a control signal which, Via lead 54, is fed back to open switchS1, to close switch S2 and to open switch S3 (while also de-energizingpower supply 40) thus de-energizing value actuator 42 so as to allowspool 17 of valve 18 to return to its lower position. At the same timeswitch S4 is opened and integrator 48 is automatically reset and madeready to start another cycle upon the next closing of switch S4.

OPERATION The operation of the detection and control circuit will beexplained with particular reference to FIG. 3 which graphicallyindicates the operational sequence of the circuit.

Initially Sl is closed, S2 is opened, S3 is closed and the spool 17 isin its lowermost position. (Shown in dashed lines.) Fluid flowing intothe valve 16 is exhausted out vent 18 and only carrier gas from line 13is supplied through the valve 16 to separator 22. As peak P1 emergesfrom chromatograph 12 and is detected by flame detector 27, the maximumdetector is poised and ready to detect the rst maximum point cz. Bydesign the ow impedance between splitter 14 and valve 16, and betweensplitter 14 and flame detector 27 is such that the maximum point a (seeFIG. 2) of GC peak P1 reaches the detector 27 slightly before the samepoint of the peak P1 reaches the valve 16. (The difference in eifectiveow distance allows for the time lag resulting from the sequence ofdetection and valve actuation.) When point a is detected a signal isgenerated by detector 36 which opens switch S1 (enabling detector 30 todetect the next minimum) and activates the power supply 40 which in turnenergizes solenoid 44 thus raising the spool 77 to its upper position.The sample flow is now being diverted through conduit 20 to separator 22where a portion thereof is separated out and introduced into massspectrometer 24.

At the same time switch S4 is closed and the integrator 48 is activated.These conditions are indicated in FIG. 3 wherein the solid linesindicate respectively, closed switches and activated components.

When a predetermined quantity of sample material (indicated by theshaded area between a and b in FIG. 2) has been allowed to pass intoseparator 22, as determined by integrator 48 and comparator 50i,amplifier 52 is activated to close switch S2, open switch S3, deactivatepower supply 40 and return valve spool 17 to its lower position openingswitch S4 and shutting down integrator 48.

When the signal from llame detector 27 reaches the point c at the end ofthe first peak "P1, the minimum detector 30 generates a signal whichopens S2 and closes S3 thus activating maximum detector 36 andcompleting a current path from the yet inactivated power supply 40 tosolenoid 44.

When the maximum detector 36 detects the maximum point d power supply 40is activated and the previously described sequence is automaticallyrepeated introducing into separator 22 the predetermined quantity ofsample material represented by the shaded area of peak P2 between pointsd and e in FIG. 2. This sequence is automatically and repeatedlyexecuted by the control apparatus so that the same quantity of samplematerial is extracted from each peak for introduction to separator 22and subsequent analysis.

As an added feature to the embodiment of the invention disclosed, it iscontemplated that a threshold detector or any other suitable detectormeans could be substituted for one or more of the detectors shown in thedrawing. Moreover, it is further contemplated that an even moreselective computing type of apparatus could be incorporated in thecircuit along with the detectors so as to further increase theselectivity of the valve control systern. It is also contemplated thatthe input to the peak detector could be taken directly from the detectorcircuitry of the gas chromatograph 12.

This sample selection and control apparatus will undoubtedly iindapplication in other analytical systems besides that depicted in thedrawing. One such application contemplated is in fraction collectionapparatus where it is desirous to be able to collect certain quantitiesof material from a gaseous mixture. Another similar applica tion isbetween a rst chromatographic column and one or more otherchromatographic columns.

After having read the above disclosure it will become apparent that manyalterations and modications may be made to the system without departingfrom the invention and it is to be understood that this description isfor purposes of illustration only and is in no manner intended to belimiting in any way and that I intend that the appended claims beinterpreted as covering all modifications which fall within the truespirit and scope of my invention.

What is claimed:

l1. In a gas handling system including, means for separating a gaseousmaterial into a series of time separated constituent peaks of differingheights carried along in a carrier gas stream, a plurality of ilow pathmeans selectively connected to said separating means by a flow directingvalve means, -means connected to at least one of said ow path means forreceiving at least a portion of said gaseous material, gas dletectormeans coupled to said separating means for detecting said constituentpeaks, valve control means responsive to said detector means to opensaid valve .means in response to the detection of predeterminedcharacteristics of said peaks, the improvement wherein said valvecontrol means further includes an integrating means responsive to saiddetector means for determining how much. separated gaseous constituentmaterial has been allowed to flow to said receiving means, said controlmeans responsive to a predetermined output of said integrator to closesaid valve means so that only a predetermined quantity of said separatedgaseous constituent material is allowed to flow to said receiving means.

2. Apparatus as recited in claim 1 ywherein said integrator means andsaid valve actuator means are operatively coupled together by acomparator means which causes a control signal to be generated `andsupplied to said valve actuator in response to a predetermined output ofsaid integrator.

3. In a gas handling system as set forth in claim 1 wherein lsaid valvecontrol means .further includes a comparator circuit responsive to said.integrating means for determining when a predetermined quantity ofgaseous constituent material has been allowed to flow to said receivingmeans.

4. yIn a gas handling system as set forth in claim 3 wherein said valvecontrol means further includes a signal detector means responsive tosaid gas detector means for rendering said integrator operative onlyafter the detection of a predetermined -peak characteristic.

S. Apparatus in accordance with claim 4 ywherein said signal 1detectormeans includes a rst means responsive to a maximum signal value and asecond means responsive to a minimum signal value.

6. In a gas handling system in accordance with claim 1 wherein saidmeans for separating the gaseous materials is a gas chromatograph andsaid gas receiving means is a mass spectrometer.

References Cited UNITED STATES PATENTS 3,301,040 1/1967 Levy et al.73-23.l '3,376,731 4/1968 Cross et al 7.3-23.1 '3,396,870 8/1968 Diamondet al. Z22-14 3,402,972 9/ 1968 Cooper et al 137-487.5X 3,405,54910/1968 Finley 73--23.1 3,049,908 8/ 1962 Kindred et al 73-23 3,421,2921/ 1969 Llewellyn 73-23.1 3,245,269 4/ 1966 Ivie 73-23.1

RICHARD C. QUEISSER, Primary Examiner E. J. KOCH, Assistant Examiner

